I'm assuming that Cadence is a Valkyrie Guardian?
No, she was never planned to be a Guardian in the first place but I’ve since changed Celestia into the leader of Mistral and as such, a main villain in this AU, so that changes what I originally planned for Cadance in this AU too.
So now she is a Host (a Host is the general term for anyone who carries the Tantabus parasite thing within them. Children are the most common Hosts but they are called Orphans instead). More specifically, she is the first successful adult Host, which is pivotal because Hosts are a necessity in this city because the parasitic entity doesn’t produce enough alicorn to supply the overwhelming demand of it from the population, and if they’re embedded into a carrier’s stomach they produce 10-15 times the amount of alicorn they would alone. Now, if a Tatabus is embedded into a living adult body, they’d produce up to 30 times the amount of alicorn, and this is why Cadance (and later Rarity) is seen as so valuable; adults don’t survive the process of injecting the parasitic entity into them, whereas children (especially pony children) typically do. There’s only 2 known adults who have successfully survived this, and that’s Cadance (being the first) and Rarity (being the second).
Anyway, Cadance: given how utterly unique she was at the time she was seen as a sort of religious figure to the Mistral population, and her face and visage was plastered all over the place and given the title “Princess”, the “key to Mistral’s future”. And she was imprisoned and drawn upon for alicorn for years until finally, finally she manages to escape with Luna’s help. They’re now in hiding somewhere in the “Underworld” (also known as Tartarus by the Mistral population and simply “Equestria” by ground-dwellers), possibly taking refuge among Shining Armor’s settlement and maybe even devising a plan to bring down Mistral to end the horrors taking place there (not to mention putting a stop to the increasingly frequent kidnappings they’ve been doing for the sake of their experiments).